The present invention concerns improvements to installations for treating water, in particular drinking water for domestic use whose quality complies with standardised characteristics that it may be desirable to improve on further, for example on an individual basis, which installations are constituted as set out in the preamble of claim 1.
Various methods are known at present using membranes to treat and purify water.
A first group of methods comprises microfiltration and ultrafiltration (referred to generically hereinafter as xe2x80x9cmembrane filtrationxe2x80x9d) respectively using membranes able to block bodies having dimensions up to 0.1 micron and up to 0.01 micron (for example filter viruses): these methods are respectively used to clarify water and to clarify and disinfect water. However, they cannot eliminate small organic or mineral molecules (salts) and further treatment may be required to obtain water of improved quality.
Membrane filtration operates at a low pressure and enables high flowrates of clarified and disinfected water to be obtained economically. The membranes are cleaned periodically by passing filtered water through them in the reverse flow direction, generally with an oxidising agent added (chlorine, hydrogen peroxide, etc.). The filtered water required for cleaning is stored beforehand in a tank and pumped back in the reverse flow direction or obtained directly from the filter modules during the operation.
Another group of methods comprises reverse osmosis and nanofiltration (referred to generically hereinafter as xe2x80x9creverse osmosisxe2x80x9d) which partly or totally eliminate small molecules and which can therefore claim to produce drinking water of improved quality. However, these methods can treat only small flowrates of water and are carried out at a high pressure: it is therefore not feasible to satisfy economically all ordinary requirements for drinking water with water treated by osmosis, in particular in an individual dwelling.
It is known that it is necessary to eliminate all particles and colloids in the water before it enters an osmosis treatment unit so as not to clog the unit irreversibly. It is also necessary to sweep the surface of the osmosis membrane to limit the concentration of species retained on the surface, too high a concentration leading to a significant drop in the rejection rate and permeability. Sweeping is often done with lost water, which leads to high water consumption.
The document DE-A-39 14 940 describes a water treatment installation in accordance with the preamble of claim 1. However, in the prior art installation, the filter means on the upstream side of the osmosis filter unit achieve insufficient filtration of the water supplying the osmosis filtered unit and the membrane thereof can clog quickly. Moreover, in an installation of the kind concerned, most of the filtered water consumed is water delivered by the first filter unit, which in the system described in the document concerned is not suitable for providing-the required flowrate of filtered water quasi-instantaneously. Finally, the installation is such that the first-filtered water and osmosis-treated water can selectively take the same portions of flow pipes, which raises the problem of pollution of the osmosis-treated water finally distributed.
The document DE-A-195 20 913 concerns an installation for producing osmosis-treated water for supplying a haemodialysis unit, in which there is on the upstream side of the osmosis filter unit a preliminary filter which can be cleaned by circulation of water in the reverse direction. However, in this prior art installation the preliminary filter can only perform coarse filtering and the water supplied to the osmosis filter unit, being insufficiently filtered, clogs the osmosis membrane very quickly. Moreover, in the prior art installation there is also the risk of contamination of the osmosis-treated water by the water that has merely undergone preliminary filtering, and the quality of the osmosis-treated water delivered may not be strictly reliable at all times.
An essential object of the invention is therefore to remedy at least some of the drawbacks of the prior art and to propose an original solution which satisfies the various practical requirements.
To this end, the invention proposes a water treatment installation as mentioned in the preamble and which is characterised by the features stated in the characterising part of claim 1.
By virtue of this improved system, the osmosis filter unit is supplied with water that has already been highly filtered by the membrane filter unit, which can advantageously be an ultrafiltration unit: the osmosis membrane therefore becomes clogged more slowly and can be cleaned effectively by circulating membrane filtered water drawn off to be consumed over its upstream surface, even if the consumption of membrane filtered water is intermittent.
Moreover, installing a reserve tank on the membrane-filtered water outlet circuit accelerates the supply of a sufficient flowrate of membrane-filtered water and allows backwashing of the membrane filter unit.
Finally, in the system according to the invention, the circuit for circulating the water for backwashing the membrane filter unit is entirely separate from the circuit supplying the osmosis filter unit and necessarily has no common point with the osmosis-treated water distribution circuit: this optimises the reliability of the production of osmosis-treated water, with no possibility of it being polluted with membrane-filtered water.
In one practical embodiment, the cleaning circuit can include a cleaning product storage tank that is in parallel with said first pipe and can be selectively inserted into the first pipe. What is more, it is advantageous for the membrane filter unit to include at least three membrane filter modules in parallel to ensure permanent availability of water at the outlet whilst providing for the maintenance and cleaning cycles.
To facilitate the operation of the installation, each end of each membrane filter module is advantageously equipped with a selection valve adapted to command in said membrane filter module a flow of water in the forward direction (filtering) or in the reverse direction (cleaning) and includes valve control means such that two membrane filter modules in parallel supply filtered water to the third module for cleaning it.
To improve the equipment of the installation, a device for coarse filtering of the water to be treated and/or a booster pump can be provided on the upstream side of the membrane filter unit.